The present invention relates generally to the field of metal finishing, more particularly to the field of the non-electrolytic immersion coating of tin-lead on metallic surfaces, and still more particularly to the provision of tin-lead coatings on selected copper surfaces as part of the fabrication sequence in printed circuit manufacture.
It has long been known that tin and lead can be co-deposited in non-electrolytic manner over metallic substrate surfaces such as copper by means of chemical exchange/replacement reaction using aqueous acidic immersion baths containing tin and lead salts. Such processes can find use in many metal finishing applications, and are in principle particularly suitable in sequences for the fabrication of printed circuits wherein tin-lead is used as an etch resist for predetermined areas of copper circuitry and/or wherein tin-lead is applied over selected copper areas (e.g., through-holes, surrounding pads, etc.) and later reflowed to provide a solder layer for preserving and/or enhancing the solderability of those areas.
The chemical reaction mechanism of tin-lead immersion plating involves dissolution/oxidation of the metal of the substrate to be plated, which in turn brings about corresponding reduction to the metallic state, and deposition on the substrate, of the divalent tin and lead in the bath. As a consequence, the tin-lead immersion plating is not autocatalytic and it is often difficult to achieve the tin-lead layer thicknesses required in many applications. Difficulty also is encountered in achieving tin-lead deposits of desired tin/lead ratio. For example, in applications where it is desired that the deposited tin-lead layer be reflowed to provide a solder layer, it is desirable that the deposited tin/lead ratio be at or near the solder eutectic (63% tin; 37% lead), but many tin-lead immersion processes tend to provide high tin content deposits irrespective of the ratio in the bath of tin and lead ions.
In arriving at the present invention, it was found that tin-lead immersion baths which produced on copper a tin-lead deposit of acceptable thickness and tin content in certain situations (e.g., on copper-clad laminate rate panels or on boards having patterned copper on epoxy/ glass laminate), nevertheless, and surprisingly, produced on copper undesirably thin tin-lead deposits, often of undesirably high tin content, in situations where the copper area was small and/or associated with surrounding areas of solder mask. This finding poses very real and practical problems, since a primary use for immersion tin-lead is in printed circuits, where small area throughholes, pads, lands, etc. are to be coated and where proximate copper traces, etc. are protected by solder mask, i.e., the very situations where the undesirable deposit characteristics are found.